Overlay target exposure device utilizing pitch determination to minimize impact of lens aberrations

ABSTRACT

A method of designing an alignment target system to minimize lens aberrations is disclosed. A first layer alignment target&#39;s pitch is selected based on the minimum feature size of the circuit. The second layer alignment target&#39;s pitch is selected based on the diffraction pattern of the first layer&#39;s target and the illumination settings of the second layer. Displacement errors are minimized when the second layer target&#39;s 1 st  diffraction order overlaps the first layer target&#39;s 0 th  diffraction order.

This application is a divisional application of U.S. patent applicationSer. No. 09/649,907, filed Aug. 30, 2000 now U.S. Pat. No. 6,432,591,the entirety of which is incorporated herein by reference.

BACKGROUND OF THE INVENTION

The invention relates to an overlay target design method forsemiconductor fabrication to minimize the impact of lens aberrations ontarget projection.

DESCRIPTION OF RELATED ART

Typically semiconductor devices are fabricated by optical lithographytechniques using a projection imaging systems. A typical projectionimage system 20 is illustrated in FIG. 1. The system 20 at a minimumincludes an illumination controller 22 and an illumination source 24coupled with and controlled by controller 22. Illumination source 24 mayinclude, for example, a mirror, a lamp, a light filter, and a condenserlens system. As used herein, the term “light” refers to light used inphotolithography. The term “light” need not be restricted to visiblelight, but may also include other forms of radiation and lithography.For example, energy supplied by lasers, photons, ion beams, electronbeams, or X-rays are included within the term “light”.

Illumination source 24 emits light or radiation that can pass throughopenings in mask 26. System 20 shows mask 26 positioned adjacent tolight source 24; optionally, other devices such as one or more opticallenses could separate light source 24 and mask 26. The term “mask” isnot limited to a physical structure, but also includes a digitized imageused in, for example, electron beam and ion beam lithography systems.For example, mask 26 may include a pattern for projecting a wiring orfeature pattern of an integrated circuit. The pattern of mask 26 mayinclude various image structures, for example, clear areas, opaqueareas, phase shifting areas, and overlay targets. Mask 26 generally is acombination of clear areas and opaque areas, where the clear areas allowlight from light source 24 to pass through mask 26 to form the mask'simage. Light passing through mask 26 is further transmitted byprojection lens 30, which may be, for example, a reduction lens or acombination of lenses for focusing the mask pattern onto a projectionsurface 111, such as a semiconductor wafer covered with a photoresistlayer. Typical semiconductor fabrication involves a four to ten timesreduction of mask size 26 by projection lens 30. Projection surface 111is held in position by a holding device (not shown), which may be partof or controlled by a stepper (not shown). Also shown in FIG. 1 is lenspupil 28 of projection imaging lens 30, which defines the numericalaperture of lens 30.

As the dimension of features on integrated circuits continue todecrease, the resolution limits of optical lithography are quickly beingreached. One limit is caused by lens aberration, which is the failure ofa lens, such as projection lens 30, to produce exact point-to-pointcorrespondence between a received image, such as from mask 26, and aprojection surface 111, such as a semiconductor die 100 a portion ofwhich is illustrated in FIG. 2. One of the many types of lensaberrations in semiconductor device fabrication is coma aberrationswhich are optical aberrations that cause the image of a mask 26 toappear comet-shaped or blurred on die surface 113 (FIG. 2). Comaaberrations result in not only line width variations and/or patternasymmetry, but also affect the location or placement of the mask imageon the die surface 113. As discussed in Takashi Saito and HisashiWatanabe's article “Investigation of New Overlay Measurement Marks forOptical Lithography,” J. Vac Sci Technol. B 16(6), November/December1998, pp. 3415, 3418, during sub-micron device fabrication using opticallithography lens aberrations cause different displacement errors foroverlay targets and device patterns.

A target is a feature on a mask 26, usually at the perimeter of the mask26, that is transferred to die surface 113 during the illuminationphase. The target helps to determine if the image transfer from mask 26to die surface 113 was properly aligned relative to lower layers.Typically, the quality of the lithographic image alignment is measuredby determining the alignment of a target on a lower level to a target onan upper or overlay level. In general the image transfer is successfulif the target of the upper-layer is approximately centered with thelower-level target. An overlay measurement system is used to measure thedistances and spaces between edges or boundaries of the upper and lowertargets. It is critically important that the circuit pattern on onelayer is accurately aligned with that of earlier layers. To evaluate thealignment of two layers, a target is formed on each layer. FIG. 2 is anillustration of a single integrated circuit (IC) die 100 fabricated on asemiconductor wafer. The locations of the electrical circuit or patternand targets are represented by large box 120, hereinafter referred to aspattern, and small boxes 110, hereinafter referred to as targets,respectively. Typical dimensions for die 100 are 5 millimeters by 5millimeters and typical dimensions for targets 110 are 10 microns by 10microns.

A prior art method of mask alignment measurement will be described withrespect to FIGS. 3 and 4. FIG. 3 is a top view of a prior art box-in-boxtarget 110 of FIG. 2. FIG. 4 is a cross-sectional view of FIG. 3 alongline III—III. The accuracy of the transfer of the targets 110approximates the accuracy of the pattern 120 transfer. A first target112 with a box pattern can be formed on surface 113 in a first layer orunder layer 115 using well known lithography techniques. Typically, asilicon oxide material is deposited over first layer 115 to form asecond layer 116. The second target 114 is formed in second layer 116with dimensions smaller than target 112 using well known lithographytechniques. The perimeter 117 of first target 112 and perimeter 118 ofsecond target 114 can be viewed by optical measurement equipment. Bymeasuring the distance between the two perimeters or isolated edges 117,118 at several locations, the center positions of targets 112, 114 canbe determined and positional deviations between the two targets 112, 114can be determined. The overlay measurement provides a comparison of thealignment of the underlay target 112 of layer 115 with that of overlaytarget 114 of layer 116.

In the article by Saito and Watanabe the benefits of using fine patterntargets made up of thin lines instead of large box shaped patterns isdiscussed. Fine patterns targets, such as targets formed with thin linewidths, are generally much closer to the actual circuit featuresdimensions than conventional large box patterns. Since lens aberrationstypically induce line width variations and create alignment errors,using fine pattern targets allows more accurate detection of lensaberrations and alignment errors. In other words, the use of the typicalbox-in-box method (FIGS. 3-4) to determine mask 26 displacement errorsis not very accurate for small device patterns, such as a quarter microndevice fabrication (0.25 micron device feature size). Using targets withfeature dimensions (size and pitch) similar to those of the circuitimproves the detection of displacement errors.

For example, FIG. 5 is an illustration of a conventional fine patterntarget system 200. Fine pattern targets 210, 220 are formed in a firstlayer 215 (FIG. 6) over surface 213. Fine pattern targets 230, 240 areformed in a second layer 216 (FIG. 6) over first layer 215. FIG. 6 is across-sectional view of FIG. 5 along line VI—VI. In known target systemssuch as target system 200 (FIG. 5), the first layer targets 210, 220 andsecond layer targets 230, 240 generally have the same pitch (P1). Theterm “pitch” refers to the distance between the outside edge of a firsttarget and the outside edge of a second target. For example in FIGS.5-6, the pitch of targets 210 and 220 are the distance between theperimeter 211 of target 210 and the perimeter 212 of target 220. Inknown target systems 200 the pitches for targets in layers 215, 216 aregenerally the same. In addition, target line widths (W1) are generallythe same for targets in layers 215, 216. However even for targets in twodifferent layers 215, 216 with the same line width W1 and pitch P1,changes in the illumination settings of light source 24, such as wavelength, intensity, and annular size, used to form the targets 210, 220,230, 240 can cause misplacement of the second layer targets 230, 240 dueto projection lens 30 aberrations.

Illumination settings are an important design factor for optimizingcircuit feature dimensions, as the settings often change from one layerto another. For example, if the second layer 216 targets 230, 240 ofFIGS. 5-6 are formed using different illumination settings than thatused for first layer 215 targets 210, 220 the light will be diffracteddifferently by the mask 26. Since the target patterns have differentdiffraction patterns, light will enter and exit lens 30 (FIG. 1) atdifferent locations. Due to normal variations in lens surfaces, if lightpasses through different locations, lens aberrations will cause thelight to diffract differently causing target displacement in the secondlayer 216.

The displacement error is a function of the mechanical placementcapability of the system 20 and the projection lens 30 aberrations. Themechanical displacement is the same for both the pattern 120 and targets110. However, lens aberrations affect the pattern 120 and targets 110differently. In most cases, the lens induced error for the pattern 120is smaller than the lens induced error in typical box-in-box targets110. The lens error is more pronounced when different illuminationshapes are used on two different layers. Since the aberrations changeacross the lens 30 the light is subject to different aberrationpatterns. Hence corrections based on typical box-in-box targets 110 orfine targets using the same pitch at both levels will inducedisplacement errors into the pattern 120.

There is a need and desire for a new method of designing featuredimensions, such as the pitch of second layer alignment targets, tominimize the impact of lens aberrations. Moreover there is a need tomaximize the lens region overlap of light diffracted from two differentillumination shapes. Furthermore, there is a need and desire for a newmethod for determining the pitch of a second layer targets based on thepitch and light diffraction patterns of a first layer target thatminimizes displacement of the second layer targets by lens aberrationsdue to changes in illumination settings.

SUMMARY OF THE INVENTION

The invention relates to a method of determining a dimension for asemiconductor feature, in particular a second layer alignment target'spitch, to minimize the impact of lens aberrations during opticalprojection. In an exemplary embodiment, the design method determines thepitch of a second layer fine pattern alignment target based on the lightdiffraction patterns of a first layer fine pattern alignment target. Thefirst layer target is designed to have a pitch similar to that of aperiodic feature of the integrated circuit, such as a capacitor. Thesecond layer target is designed to have a pitch that minimizesdisplacement of the second layer target by optimizing the lightdiffraction patterns of the second layer target based on the first layertarget.

The pitch of the second layer target is determined by several steps.First, projection lens locations of the light diffraction patternscreated by the first layer target for a particular illumination settingare determined. Second, the projection lens locations of the 0^(th)order light diffraction pattern for the second layer illuminationsettings are determined. Finally, the second layer target's pitch isselected which optimizes overlap of the 1^(st) order light diffractionpatterns of the second layer target with that of the 0^(th) orderdiffraction pattern of the first layer target. The more overlap betweenthe respective diffraction patterns of the first and second layertargets, the more the displacement error caused by lens aberrations isreduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The above issues and other advantages and features of the invention willbe more readily understood from the following detailed description ofthe invention provided in connection with the accompanying drawings.

FIG. 1 is an illustration of a conventional optical imaging projectionsystem;

FIG. 2 is an illustration of conventional locations for electricalcircuit patterns and alignment targets for an integrated circuitfabricated on a semiconductor substrate;

FIG. 3 is a top view of a conventional box-in box target system;

FIG. 4 is a cross-sectional view of FIG. 3 along line III—III;

FIG. 5 is a top view of a conventional fine pattern target system;

FIG. 6 is cross-sectional view of FIG. 5 along line VI—VI;

FIG. 7 is a top view of a fine pattern target system according to anexemplary embodiment of the present invention;

FIG. 8 is a cross-sectional view of FIG. 7 along line VIII—VIII;

FIG. 9 is a cross-sectional illustration of an imaging system used forforming the first layer targets of FIGS. 7-8;

FIG. 10 is a cross-sectional illustration of the projection lenslocation of the 0^(th) order light diffraction pattern for the firstlayer targets of FIGS. 7-8;

FIG. 11 is a cross-sectional illustration of the projection lenslocation of the −1, 0^(th), and 1^(st) light diffraction patterns forthe first layer targets of FIGS. 7-8;

FIG. 12 is a cross-sectional illustration of an imaging system used forforming the second layer targets of FIGS. 7-8;

FIG. 13 a cross-sectional illustration of the projection lens locationof the 0^(th) order light diffraction pattern for the second layertargets of FIGS. 7-8;

FIG. 14 is a cross-sectional illustration of the projection lenslocation of the −1, 0^(th), and 1^(st) light diffraction pattern for thefirst layer targets of FIGS. 7-8 and the 0^(th) light diffraction ordersof the second layer targets of FIGS. 7-8;

FIG. 15 is a flow chart of the design method for determining the pitchfor second layer targets of FIGS. 7-8 according to the presentinvention;

FIG. 16 is a cross-sectional illustration of the projection lenslocation for light diffraction patterns of the second layer targets ofFIGS. 7-8 which optimize the overlap of the 1^(st) order diffractionpattern of the second layer target with the 0^(th) order diffractionpattern of the first layer target according to the method of the presentinvention; and

FIG. 17 is a graphical bar chart comparison of the displacement errorfor various combinations of aberration coefficients for a conventionalfine pattern target compared to a fine pattern target designed accordingto a method of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention will be described as set forth in the exemplaryembodiments illustrated in FIGS. 7-16. Other embodiments may be utilizedand structural and functional changes may be made without departing fromthe spirit or scope of the present invention.

FIGS. 7-8 are illustrations of an exemplary target system 600 designedaccording to a method of the present invention as described in regardsto FIGS. 9-16. FIG. 8 is a cross-sectional view of FIG. 7 along lineVIII—VIII. The fine pattern targets 610, 620 of first layer 615 areformed on layer 613. The targets 610, 620 are shown as two concentricgeometric squares. It is to be understood that other shapes, number oftargets, and arrangements are possible options, if so desired. The firstlayer targets 610, 620 are separated by distance P2, hereinafter calledpitch P2. Ideally, pitch P2 should be close to the circuit's limitingdimension, such as the spacing between capacitors in a DRAM device. Forexample, if the minimum device dimension is 1 micron, the first layertarget pitch P2 should be close to 1 micron as well.

In an exemplary embodiment, as described in FIGS. 9-16, the pitch P2 oftarget 610, 620, is selected at 1 micron to approximate the dimension offorming a trench in an insulating layer. The pitch P2 is the distancefrom the perimeter 611 of the outside first layer target 610 and theperimeter 612 of the inside first layer target 620. The pitch P2consists of the combined distance of line width W2 and space S2. Theline width W2 and space S2 are selected at 0.5 microns each. It is to beunderstood that the selection of P2, W2, and S2 can vary withoutlimiting the scope of the invention.

Referring to FIG. 8, a second layer 616 is shown formed over layer 615with second layer targets 630, 640. The pitch P3, line width W3, andspace width S3 of second layer targets 630, 640 are shown. In theexemplary design method of the present invention (as discussed in FIGS.9-16), the pitch P3 is designed to minmize lens aberrations or imagedisplacement errors caused by projection lens 30. It is to be understoodthat the shape and number of fine patterns is not to be limited by theexemplary embodiment. Two concentric boxes made up of thin lines are butone of numerous configurations for alignment targets.

FIG. 9 is a cross-sectional illustration of an annular light source 800used to transfer an image of mask 810 containing first layer targets610, 620 of FIGS. 7-8. The light source 800 of the exemplary embodimenthas a wavelength (λ2) of 248 nanometers. The outer illumination diameter801 represented by fine L2 is 0.8 numerical aperture units (N.A. units).The inner illumination diameter 802 represented by L1 is 0.5 numericalaperture units. The image of mask 810 is illuminated by light source 800and reduced by projection lens 815 before being transferred to a wafer(not shown). The projection lens 815 has a numerical aperture 806 of0.63 N.A. units. The lens axis 805 is shown by the center line.

FIG. 10 is a cross-sectional illustration of the projection lens 815location of the 0^(th) order light diffraction patterns 820 generated bylight source 800 passing through mask 810 containing first layer targets610, 620 of FIGS. 7-8. The diffraction order locations for projectionlens 800 can be determined using an imaging software, such as Sigma C,manufactured by Solid C Corporation of California, using known lightdiffraction equations. The location of the 0^(th) order region isrepresented by the two blocks 820. The 0^(th) diffraction orderrepresent regions where light was not deviated by the is first layertargets 610, 620 of mask 810. Blocks 820 are located between 0.315 and0.501 N.A. units from the lens axis 805. The 0^(th) diffraction regionis 0.189 N.A. units wide. The two boundaries are established by lightfrom the outer 801 and inner 802 diameters of the light source 800passing through mask 810. The light paths are illustrated by lines 803,804.

FIG. 11 is a cross-sectional illustration of the projection lens 805location of the 0^(th) diffraction order 820, the 1^(st) diffractionorders 830, and the −1 diffraction orders 840 of first layer targets610, 620 formed on mask 810. Part of the 1^(st) diffraction order islocated between 0.067 and 0.256 N.A. units from the lens axis 805.Another part diffraction order is located at 0.563 and falls partiallyoutside the lens aperture. It is to be understood that the presentinvention can be used to analyze any diffraction order and is notlimited to the 0^(th) and 1^(st) order diffraction patterns.

The distance D1 between the boundaries of the 0^(th) order diffractionregions 820 and the 1^(st) order diffraction regions 830 is a functionof the light wavelength λ2 and first layer target pitch P2. Distance D1equals the wavelength λ2 divided by pitch P2:

D 1=λ2/P 2 or 248 nm/1000 nm=0.248 numerical aperture  (1)

FIG. 12 is a cross-sectional illustration of conventional light source900 used to transfer an image of mask 910 containing second layertargets 630, 640 of FIGS. 7-8. The light source 900 of the exemplaryembodiment has a wavelength λ3 of 248 nanometers and diameter of 0.305microns. The diameter is represented by line L3 and the light source 900has an illumination diameter of 0.192 N.A. units. The image of mask 910is illuminated by light source 900 and is reduced by projection lens 815which was previously described above. Typically the same projection lens815 is used for the first 810 and second 910 masks.

FIG. 13 is a cross-sectional illustration of the projection lens 815location of the 0^(th) order light diffraction patterns 920 generated bylight source 900 passing through mask 910. The 0^(th) diffraction orderlocations for projection lens 815 can be determined as discussedpreviously. The location of the 0^(th) order region is represented bythe block 920. Block 920 is between the lens axis 805 and 0.192 N.A.units from the lens axis 805. The light paths are illustrated by lines901, 902 travel from light source 900 through mask 910 to projectionlens 815. The lens location of the 0^(th) diffraction pattern isindependent of the pitch P3 of the second layer targets 630, 640. FIG.14 is a cross-sectional illustration of the diffraction patterns 820,830 (FIG. 11) of first layer targets 610, 620 superimposed over the0^(th) diffraction 920 pattern of the second layer light source 900 asdiscussed in FIG. 13. The distance D2, which equals 0.507 N.A. units,represents the distance between the 0^(th) diffraction orders 820, 920of the first and second layer targets 610, 620, 630, 640.

To minimize the misalignment of the second layer targets 630, 640 withthe first layer targets 610, 620, the pitch P3 of the second layertargets 630, 640 should be designed to minimize lens aberrations. Anexemplary method for determining the pitch P3 for second layer targets630, 640 based on the light diffraction patterns of the first layertargets 610, 620 is described in FIG. 15 (process segments 520, 530,540, 550). The first segment 520 of the method 500 is to select a pitchP2 for first layer targets 610, 620. The second segment 530 is todetermine the projection lens locations of the diffraction ordersgenerated from first layer targets 610, 620 as described in FIGS. 10-11.The third segment 540 is to determine the distance D2 between the firstlayer targets 610, 620 0^(th) diffraction orders 820 and the secondlayer targets 630, 640 0^(th) diffraction order 920. The distance D2 canbe solved using, for example, the graph in FIG. 14. The fourth segment550 is to select a pitch P3 for the second layer targets 630. 640 sothat the majority of the 1^(st) light diffraction order for the secondlayer targets 630, 640 passes through the projection lens 30 (FIG. 1) inthe region where the 0^(th) light diffraction order for the first layertargets 610, 620 passed.

An exemplary method is to select a pitch P3 whereby the distance D3,between the 0^(th) diffraction order 920 and the 1^(st) diffractionorder 930, is equal to D2. It is to be understood that other methodscould result in more optimal overlaps, the above method does not limitthe scope of the invention. 0ther methods to include projection imagingsoftware or trial and error methods, could be used to analyzediffraction patterns for various pitches P3 to select pitch P3 forsecond layer targets 630, 640, which produce a 1^(st) diffraction order930 that maximizes overlap with the 0^(th) diffraction order 820 or the1^(st) diffraction order 830 of the first layer targets 610, 620. It isto be understood that the invention is not limited to this. For example,another method is to to maximize the overlap of the 0^(th) diffractionorder 920 of the second layer 616 with the 0^(th) diffraction order 820or 1^(st) diffraction order 830 of the first layer 615.

FIG. 16 is a cross sectional illustration of the exemplary method ofstep 550 where distance D3 is selected to equal D2 which is 0.507 (FIG.14). The first diffraction order 930 of second layer targets 630, 640begins at 0.315 N.A. units from the lens axis 805 and extends past thelens numerical aperture 806. The distance D3 between the 0^(th) orderdiffraction pattern 920 and the 1^(st) order diffraction pattern 930 is0.507 N.A. units. Thus pitch P3 of target 630, 640 can be solved usingthe equation:

P 3=(λ3/D 3)=248 nanometers/0.507=0.489 microns  (2)

In most circumstances exact overlap will not be possible, so pitch P3will be designed to minimize the amount of displacement error for agiven illumination setting.

FIG. 17 is a bar graph comparison of the displacement errors generatedby lens aberrations for a conventional target system 200 versus thetarget system 600 of the present invention. The bars labeled 710represent the lens displacement errors for prior art target systems 200for various combinations of light aberration coefficients. The barslabeled 720 represent the lens displacement errors for target systems600 formed according to the present invention. The use of variousaberration coefficients to determine the refracting properties of a lensare known in the art. FIG. 17 illustrates the impact that variouscombinations of aberration coefficients have on the displacement of thetarget systems 200, 600. At combinations 11 and 12 the displacementerror for target system 200 exceeds 30 nanometers, while thedisplacement error for target system 600 was reduced by over 50%.

Having thus described in detail exemplary embodiments of the invention,it is to be understood that the invention defined by the appended claimsis not to be limited by particular details set forth in the abovedescription as many apparent variations thereof are possible withoutdeparting from the spirit or scope of the invention. Accordingly, theabove description and accompanying drawings are only illustrative ofexemplary embodiments which can achieve the features and advantages ofthe present invention. It is not intended that the invention be limitedto the embodiments shown and described in detail herein. The inventionis only limited by the scope of the following claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A projection imaging system comprising: anillumination source; a mask, said mask comprising at least one target,wherein said target is formed by: determining light diffraction patternsof a lower layer target and selecting a pitch for the mask targetwherein at least a 1^(st) order diffraction pattern of said mask targetoverlaps at least some portion of a 0^(th) order diffraction pattern ofsaid lower target; a projection lens; and a projection surface.
 2. Theprojection imaging system of claim 1, wherein the projection surface isa substrate.
 3. The projection imaging system of claim 1, wherein themask target are fine pattern targets.
 4. The projection imaging systemof claim 1, wherein the projection surface is coated with a photoresistmaterial.
 5. The projection imaging system of claim 1, wherein the maskcomprises and integrated circuit fabrication pattern.